1. Field of the Invention
The present invention relates to an image forming apparatus such as an electrostatic copying machine, an electrostatic printer and the like, in which a toner image is transferred from an image bearing member onto a transfer material.
2. Related Background Art
There have widely been used image forming apparatuses for transferring a transferable toner image formed on an image bearing member onto a transfer material.
FIG. 6 is a schematic constructural view showing a typical example of such an image forming apparatus. Briefly explaining construction and function of the image forming apparatus, a surface of an image bearing member (referred to as "photosensitive member" hereinafter) 10 rotated in a direction shown by the arrow X is uniformly charged by a first charger 11, and, by applying image exposure 12 (such as a laser beam which is image-modulated) to the charged surface of the photosensitive member, potential of the exposed surface is reduced, thereby forming an electrostatic latent image.
Then, when the latent image reaches a developing station where the photosensitive member 10 is opposed to a developing device 13, toner is supplied to the latent image from the developing device 13, thereby forming a toner image. During further rotation of the photosensitive member 10, when the toner image reaches a transfer station where the photosensitive member 10 is opposed to a transfer charger 16, in synchronous with this, a transfer material 18 conveyed and guided by a transfer guide member (referred to as "transfer inlet guide" hereinafter) 114 comprised of an upper transfer material guiding member (referred to as "transfer inlet upper guide") 114a and a lower transfer material guiding member (referred to as "transfer inlet lower guide") 114b is supplied to the transfer station to be contacted with the photosensitive member 10, and the toner image on the photosensitive member 10 is transferred onto the transfer material 18 by the action of a transfer electric field generated by the transfer charger 16.
Thereafter, the transfer material 18 is separated from the photosensitive member 10 and then is conveyed to a fixing station (not shown), where the toner image is fixed to the transfer material. Then, the transfer material is discharged out of the image forming apparatus. On the other hand, residual toner remaining on the photosensitive member 10 is removed by a cleaner 17 for preparation for next image formation.
The transfer inlet guide 114 is sometimes formed from insulation material, so that poor transferring such as transfer void (generated by leaking transfer current to the transfer inlet guide 114 through the transfer material which absorbed moisture to reduce the resistance thereof under high humidity environment) can be prevented.
Recently, a transfer charger of contact type such as a transfer roller has widely been used as the transfer charger. In comparison with a corona charger, the transfer charger of contact type has advantages that capacity of a power source can be reduced and that discharge products such as ozone can be suppressed.
As an image forming method, normal development in which only a portion of the charged surface of the photosensitive member corresponding to a background portion of the image information is exposed (background exposure system) and portions other than the background portion is developed, or inversion-development in which only a portion of the charged surface of the photosensitive member corresponding to an image information portion is exposed (image exposure system) and non-exposed portion is developed can be used.
However, in the arrangement as shown in FIG. 6, the transfer inlet guide 114 is expected to positively direct the transfer material 18 to a predetermined position on the photosensitive member 10. Thus, a tip end (near the photosensitive member) of the transfer inlet guide 114 is normally disposed in the vicinity of the surface of the photosensitive member 10 with a small gap of about 1 to 3 mm, and, since the transfer inlet guide 114 is disposed relatively near the transfer charger 16, the transfer inlet guide is apt to be charged with polarity same as the transfer current, i.e., polarity opposite to that of the toner.
Accordingly, the toner floating within the apparatus and/or the toner on the surface of the photosensitive member 10 at the transfer station is electrostatically absorbed to the transfer inlet guide (particularly, the transfer inlet upper guide 114a positioned at an upstream side in a photosensitive member moving direction) to contaminate the guide. In particular, a relatively large amount of toner is adhered to the tip end of the guide, with the result that the toner is adhered to the transfer material to deteriorate image quality.
Particularly, in the inversion-development in which the toner image is formed on the surface of the photosensitive member having the same polarity as that of the toner, since an adhering force of the toner to the surface of the photosensitive member is small in comparison with the normal development, the toner is more apt to be absorbed to the transfer inlet upper guide 114a electrostatically.
To avoid this, if the transfer inlet upper guide 114a is spaced apart from the surface of the photosensitive member, a curled transfer material cannot be conveyed to the transfer station positively, thereby causing poor conveyance (sheet jam).
In addition to the above-mentioned inconveniences, when a special transfer material is used, the following disadvantage occurs.
That is to say, when a transfer material having relatively great resiliency such as a post card and an envelope is used, after the transfer material is passed through a pair of regist rollers 15, the trail end of the transfer material is sprung up to contact with or strike against the transfer inlet guide 114, with the result that the toner adhered to the transfer inlet upper guide 114a is scattered by the shock, thereby contaminating an imaged surface of the transfer material.
Particularly, in the inversion-development, since the adhering force of the toner to the surface of the photosensitive member is small in comparison with the normal development, the toner is influenced by a small electrostatic force, so that not only the toner is easily adhered to the transfer inlet upper guide 115a but also the image is apt to be subjected to an image scattering phenomenon such as smeared image trailing edge. It is considered that the image scattering phenomenon is generated by distorting the toner image by a weak transfer electric field existed immediately in front of the transfer station (referred to as "pre-transfer" hereinafter).
In order to prevent the image scattering phenomenon, the transfer inlet guide 114 may be disposed at a relatively upstream side of the surface of the photosensitive member so that the transfer material is contacted with the surface of the photosensitive member at a position sufficiently spaced apart from the transfer station, i.e., in front of a pre-transfer area. However, in such an arrangement, an amount of spring-up of the trail end of the transfer material having relatively great resiliency is further increased.